The invention relates to the positioning of a glass sheet travelling on a conveyor, particularly a glass sheet heated to beyond its softening point with a view to its cambering and/or a heat treatment intended to modify certain of its mechanical characteristics. The invention more particularly relates to the production of glass plates or windows for motor vehicles. 2. Background of the Related Art
In order to supply a glass sheet which satisfies the important requirements of good conformity of camber and a satisfactory optical quality, it is important for the glass sheet to be correctly positioned prior to being taken up by the cambering and/or heat treatment tools. However, a glass sheet heated to beyond its softening point cannot be held by grippers or other gripping members, unless one is willing to accept the marks necessarily left behind by these members. Moreover, the positioning of the glass sheet relative to the tools prior to heating it, i.e., immobilizing these tools for an exceptionally long period, leads to very high production costs.
These problems of gripper marks and the stopping of the tools are significantly reduced with cambering and/or heat treatment processes in which the first stage, namely the heating of the glass sheet, takes place in a furnace traversed by the glass sheets, e.g., conveyed on a roller bed. However, the positioning problem is not solved because the travel speeds and the lengths of the furnaces are such that two successive glass sheets do not always have strictly identical trajectories. The trajectory variations can result in two types of defects, namely sliding perpendicular to the conveyor axis and rotation with respect to said axis and which are respectively obviated by lateral positioning means and frontal positioning means.
As stated in European Patent application No. EP-A-267 120, the lateral positioning may be obtained by means of guide bars which move together when advancing in the forward movement direction of the conveyor and which define a passage for the glass sheet. The glass rubs against the bars, but is not stopped by them.
However, as the glass sheets intended for motor vehicle windows are never in the form of rectangular plates, but almost always have a width varying from one end to the other of the plate, said lateral positioning is not adequate and must be completed by a frontal positioning which, according to EP-A-No. 267 120, involves a glass sheet shopping phase. In order to give the glass sheet the desired orientation, a stop member, e.g., one constituted by two abutments, is positioned along its path and is necessarily struck by one end of the leading edge of the sheet and maintained in place for an adequate time to enable the glass to be reoriented by the conveyor, which continues to move the glass sheet. For as long as the glass sheet is at least partly immobilized by the stop member, it rubs at the same point on the rollers, which can then leave behind an impression on the glass which is very easily marked due to its high temperature. Thus, frontal positioning is the source of numerous optical defects.